1. Field of the Invention
The present invention relates generally to millimeter wavelength mixers and, more particularly, to millimeter wavelength mixers which provide output signals derived from higher-order energy modes of the signals applied thereto.
2. Description of the Prior Art
Balanced mixers have long been well known in the prior art relating to waveguide structures, as evidenced by the patents referenced herein. For example, Pritchard et al in U.S. Pat. No. 2,834,876 disclose a balanced mixer using a directional coupler for mixing microwave energy. The disclosed device specifically provides for control of reflected energy by strategic positioning of crystal diodes. The Curtis U.S. Pat. No. 2,790,073 discloses a balanced mixer arrangement for microwave systems employing antireciprocal circulators, such as ferrites for switching and duplexing. The Riblet U.S. Pat. No. 2,568,090 describes a balanced mixer for a radar system. The balanced mixer utilizes directional couplers and is particularly designed to develop two differently phased output voltages for direct use in push-pull application. The Fernsler U.S. Pat. No. 2,550,409 discloses a balanced mixer for controlling the phase of respective signals so that mutual cancellation of microwave energy fields within a waveguide cavity may be achieved. The Arnold U.S. Pat. No. 2,686,260 discloses structure having dual mixers within a single waveguide, and is directed to developing means for preventing cross-coupling of energy between the two mixers. The McClain, Jr. U.S. Pat. No. 2,605,400 discloses a superheterodyne receiver for radio frequencies in the range of 10 Ghz. This receiver includes a single crystal detector mounted within a waveguide coupler extending between the antenna and the first detector. McCoy et al. in U.S. Pat. No. 2,813,973 disclose a parallel waveguide structure having a single transmission line which is coupled to both waveguides for signal transfer. This structure utilizes a crystal diode for frequency conversion.
The Riblet U.S. Pat. No. 3,263,176 discloses a waveguide structure for equally dividing input microwave energy between two parallel microwave transmission paths. A cavity resonator, set at a reference frequency, is coupled in equal degree to the two transmission paths to develop wave energy with 90.degree. phase shift in the two paths. The Forrer et al patent 3,046,497 discloses a waveguide balanced modulator for operation at 3 Ghz. A pair of crystal diodes is employed, each being coupled between a corresponding one of a pair of coaxial transmission lines and a corresponding waveguide element. However, two independent waveguide sections are utilized, and overmode mixing is not contemplated.
Current state of the art, millimeter-wavelength, balanced mixers are fabricated from sections of waveguide with inside dimensions as small as 0.034.times.0.017 inches at 300 Ghz. These small dimensions pose many problems in the construction of mixers, both from the standpoint of machining such small dimensions and in mounting semiconductors within those waveguide dimensions in order to extract energy from the waveguide. In addition to the mechanical problems, the loss of energy confined within these dimensions is very high. In conventional balanced mixers, the signal energy and local oscillator energy must be equally divided so that the energy may be applied to separate extraction diodes 180.degree. out of phase with each other. This process requires a very complex and lossy structure such as a "magic T" coupler, sidewall coupler or directional coupler.
Therefore, it would be an improvement in the microwave mixing art to provide for a mixer which may be utilized at millimeter wavelengths but which has mechanical dimensions sufficient to provide for easy machining and mounting of energy extraction diodes. In addition, it would be an improvement in the mixer art to provide a mixer which has lower energy losses as compared with conventional mixers.